Process for producing non-aging steels



Oct 24, 1967 J. F. ENRIETTO 3,348,980

PROCESS FOR PRODUCING NON'AGING STEELS Filed Mafch 9, 1965 2 Sheets-Sheet 1 SOAK TIME (HR) ANNEALING TEMP. (FI

THEORETICAL SOAK TIME NECESSARY TO REDUCE THE NITROGEN CONCENTRATION FROM 0.004% T0 0.000270 j AS A FUNCTION OF ANNEALING TEMPERATURE. H FLOW RATE= 2500 CFH. SHEET THICKNE$S= 0.035'.

MASS OF STEEL GIVEN 0N CURVES.

FIG. I.

INVENTOR Joseph E Enrieflo Get. 24, 1967 Filed March 9, 1965 J. F. ENRIETTO PROCESS FOR PRODUCING NON-AGING STEELS 2 Sheets-Sheet 2 OPTIMUM ANNEALING TEMPERATURE As AIFUNCTION F cou. SIZE.

I? HYDROGEN FLOW RATE 2000-5000 CFM.

SHEET THICKNESS- 0.036 in.

LL! II 2 (I022- 0: LL] 0. z w 976 D.-

\0 so so COlL MASS (I000 lb.) FIG. 2.

HYDROGEN FLOW RATE=40 00mm. AGING INDEX AS A STRIP TH|CKNE$S=0.036" FUNCTlON OF ANNEALING ANNEAUNG TIME=IO HOURS TEMPERATURE. DOTS-ACTUAL EXPERIMENTAL RESULTS 7; 0000 $01.10 LINE CURVE-CALCULATED. D.

X m 6000 .0006 5 g 8 0 4000 .00025 E i, E E 11. o ,3 2000 .00015 im 5 z 0- A -.000| 932 I022 I112 I202 I292 TEMPERATURE F. FIG. 3.

INVENTOR Joseph F. Enriefio United States Patent Ofifice Patented st. 24, 1967 3,348,980 PROCESS FOR PRQDUCING NON-AGING STEELS Joseph F. Enrietto, Pittsburgh, Pa., assignor to Jones &

Laughiin Steel Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Mar. 9, 1965, Ser. No. 438,241 4 Claims. ((31. 148-16) ABSTRACT OF THE DISCLOSURE This patent discloses production of non-aging steels by opened-coil annealing in dry hydrogen at between 975 F. and 115 0 F. to obtain rapidly a low nitr gen content of 0.0005% max. by weight.

This invention pertains to the production of non-aging steels, and more particularly to the production of nonaging rimmed steel by annealing in hydrogen.

As is known, one of the characteristics of a rimmed low carbon steel is its susceptibility to the phenomenon known as strain aging. Strain aging can be defined as any change in the mechanical properties of a steel which takes place with time after the steel has been plastically deformed. The deformation can be imposed by temper rolling, tension, bending, drawing or other cold work; and the changes may take the form of a return of the yield point, an increase in the flow stress or hardness, or a loss in ductility.

It is known that strain aging is caused by the presence f interstitial atoms of carbon or nitrogen in solid solution in sufiicient quantity to pin or lock the dislocations produced by plastic deformation so as to render them less mobile. In this respect, it can be said that the presence of nitrogen or carbon above a'predetermined maximum limit in solid solution in a-il'On at a temperature willciently high to render the interstitial atoms mobile enough to diffuse to dislocations is a necessary condition for strain aging.

In order to prevent strain aging, either (1) the carbon and nitrogen atoms in solid solution must be immobilized so that they cannot easily migrate to disclocations, or (2) the interstitial concentration of nitrogen or carbon in solid solution must be reduced to less than some minimum value. Considering, first, the problem of immobilizing carbon and nitrogen atoms while retaining them in solid solution, the most obvious method of control is simply the use of temperature. Unfortunately, however, the diffusion coefficients of these elements are so large that even at room temperature strain aging can take place within three weeks. Since it is impractical to store substantial amounts of steel at subfreezing temperatures for long periods of time, the control of strain aging by means of temperature is entirely impractical for commercial applications.

The second alternative to reducing the mobility of the interstitials is to remove them from solid solution altogether either by precipitating them with the addition of alloying elements or chemically taking them completely out of the steel as by decarburizing or denitriding the steel. Although reports of experiments fixing the amount of free carbon and nitrogen necessary to produce strain aging are few, it has been estimated that 0.000l% by weight carbon or nitrogen will produce detectable aging and that 0.001% by weight will produce severe aging. The equilibrium solubility of both Fe N and Fe C falls far below 0.000l% by weight at room temperature; yet mild steels still exhibit aging. Carbon strain aging at concentrations above about 0.04% by weight can be eliminated by rapidly cooling the steel through the transformation range to cause a fine carbide distribution, and then slow cooling to about 300 F. to allow precipitation. Since the control of temperature in this manner for the prevention of carbon strain aging can easily be controlled commercially, carbon need not be responsible for strain aging. The behavior of nitrogen, on the other hand, is quite different. Because it is usually present in concentrations of less than 0.01% and is much more soluble than carbon, it is virtually impossible to precipitate sufiicient nitrogen as Fe N to prevent strain aging. That is, there is always a high degree of super saturation.

In order to precipitate nitrogen, it is necessary to add an alloying element which will either form a stable nitride or cause the Fe N to precipitate to equilibrium. Aluminum, silicon and boron all form stable nitrides and their addition to low carbon steels can render the steel non-aging. Unfortunately, however, the addition of the two former elements kill the steel because of their strong affinity for oxygen; and the addition of boron necessitates-special equipment and control to prevent it from preferentially combining with oxygen. The addition of weaker nitride formers such as vanadium has also been attempted to prevent strain aging. The use of vanadium, however, is prohibited by the cost of the alloying element and annealing cycles which are needed for its successful operation. Thus, the employment of alloying elements as a practical means of denitriding for commercial applications has not found acceptance.

Finally, there remains the method of removing nitrogen from the steel 'by annealing the strip in a special atmosphere; and the present invention has as its primary object the provision of such a method. Specifically, the invention is concerned with a method for denitriding rimmed steels with the use of hydrogen in an open coil anneal, in which hydrogen forms ammonia gas in its reaction with the nitrogen in the steel.

As will be seen, the invention resides in the realization of the fact that by open coil annealing coils of low carbon rimmed steels weighing in the range of about 10,000 to 80,000 pounds within upper and lower temperature limits, the nitrogen content of the steel can be reduced to at least 0.000S% by weight and in many cases to less than 0.0002% by weight. A nitrogen content of less than 0.0005%, however, is sufficient for a commercially acceptable non-aging steel. By annealing the strip material in a dry hydrogen atmosphere at ordinary annealing temperatures in the range of about 1200 F. to 1300 F., the process takes an extraordinarily long time, thereby making it economically unfeasible. However, contrary to what might be expected, the process can be hastened so as to make it economically practical by annealing at a lower temperature within the range of about 975 F. to 1150 F. Below about 975 F., the action will again take a long time.

Thus, in accordance with the present invention, it has been found that there is a critical temperature range at which the soak time will be a minimum. The specific optimum temperature within the aforesaid range will depend upon the thickness of the sheet, the mass of the steel within the annealing furnace and the flow rate of hydrogen through the furnace. This optimum temperature can be theoretically calculated, as will be seen. The essential point in practicing the invention is that the annealing time and temperature be near that which is calibrated from the equations which will be given later if optimum results are to be obtained. Once the coil mass, the strip thickness and the hydrogen flow rate have been set, the annealing temperature can be determined.

In practicing the invention, the steel in strip form is loosely coiled to permit hydrogen gas to flow along the surfaces of all the convolutionsin the coil, and the coil placed in a batch annealing furnace. Dry hydrogen gas is caused to flow through the annealing furnace and along the surfaces of the strip between successive convolutions of the coil, while the temperature of the furnace is maintained somewhere in the range between 975 F. and 1150 F., the exact optimum temperaturebeing calculated from the mass of the coil, the thickness or gage of the strip and the flow rate of hydrogen through the furnace as mentioned above. If the mass of the coil or the thickness of the sheet is increased, the optimum annealing temperature also increases. On the other hand, if the sheet thickness or coil mass is decreased, the optimum temperature also decreases. In a similar manner, if the flow rate of hydrogen through the furnace is increased, the

optimum annealing temperature decreases. In all cases, it is necessary to provide a hydrogen flow rate suflicient to achieve complete mixing of the hydrogen gas and the ammonia formed at the surfaces of the strip. Otherwise, the accumulation of nitrogen at the surfaces will impede the reaction of hydrogen with nitrogen in the strip.

The foregoing and other objects of the invention will become apparent from the following. detailed description taken in connection with the accompanying drawings, in which:

FIGURE .1 is a graph illustrating the annealing or soak time necessary to denitride coils of steel of different weights in accordance. with the teachings of the invention;

FIG. 2 is a graph illustrating the manner in which the optimum annealing temperature varies as a function of coil mass; and

FIG. 3 comprises a graph showing the manner in which the aging index and nitrogen content of the steel vary as a function of annealing temperature.

While the invention is not limited thereto, experiments conducted to establish the new and improved results of the invention utilized commercial quality rimmed steels which were cold-rolled to twenty gage (0.036 inch) after being processed from the ingot stage in the usual manner and having the nominal composition shown in Table I.

TABLE I [Typical chemical analysis of rimmed steels] C Mn P 5 Cu Ni Cr M0 N Note that the nitrogen content of the steel is 0.004%, far above the 0.0005 necessary for a commercially acceptable n-on-aging steel.

In Table II, the effect of dry hydrogen on the aging index of steel having the composition shown in Table I is illustrated.

l Annealing temperature=1,112 F.; samples water quenched from 1,292" I before heat treatment.

2 Difference in flow stress after 12% prestrain between aged and unaged sample.

As indicated in Table II, aging index is defined as the difference in flow stress after 12% prestrain between the aged and unaged sample. The samples utilized to obtain the tabular results of Table II were denitrided by annealing in flowing dry hydrogen at a temperature of 1112 F. for the times indicated. The results of Table II show that it is possible to produce a non-aging steel by annealing in hydrogen so as to remove only the nitrogen. Moreover,

the process is time dependent, but still takes place rapidly at 1112 F.TO make the steel completely non-aging, it is necessary to reduce the nitrogen to about 0.0001% as shown in Table II; however a nitrogen concentration of 0.0005 is considered to be commercially acceptable as mentioned above. Note the extreme sensitivity to nitrogen concentration. Thus, 0.00035% nitrogen gives an aging index of 6600 psi. compared to 0 for a concentration of 0.0001%. The maximum aging index measured as shown by Table II was 9100 p.s.i-., which corresponded to 0.0015% nitrogen in solid solution, above the commercially accepted minimum.

FIG. 1 shows the theoretical soak time necessary to reduce the nitrogen concentration from 0.004% to 0.0002% as a function of annealing temperature for steel coils open coil annealed and weighing 40,000, 60,000 and 80,000 pounds, respectively. The hydrogen flow rate under these conditions is 2500 cubic feet per hour at a sheet thickness of 0.035 inch. Note that the soak time increased for all coils beneath a temperature of 975 F., and above 1150 F. Thus, there is a critical temperature range within which annealing or soak time .is a minimum. As mentioned above, the optimum temperature depends primarily on the thickness of the sheet, the mass of the steel and the flow rate of hydrogen through the furnace. From this information, the optimum temperature can be calculated by means of the following two differential equations:

[N ]=The nitrogen concentration in weight percent at the surface of the sheet.

F=The flow rate of hydrogen into the furnace.

M =The mass of steel in the furnace.

[N,,]=The average nitrogen concentration in the coil.

t=Time.

V==Volume of the furnace.

R=Gas constant.

T=Temperature.

D=Diffusion coelficient of nitrogen in iron.

I=Thickness of the sheet.

[N I]1=Nitrogen concentration at the. 1 position in the s eet.

By solving these equations, one can obtain the time necessary to reduce the nitrogen concentration to any value as a function of temperature; and while the exact optimum temperature will be dependent upon the thickness of the sheet, mass of the steel and the flow rate of hydrogen through the furnace as mentioned above, it will always be within the range of 975 F. to 1150 F. for coil sizes in the range of 10,000 to 80,000 pounds, beneath the ordinary annealing temperature of about 1200 F. to 1300" F.

FIG. 2 shows how the optimum annealing temperature varies with coil size. This particular curveis applicable for hydrogen flow rates of 2000 to 5000 cubic feet per hour..If the flow rate exceeds 5000 cubic feet per hour, the curve will be slightly displaced toward higher temperatures; the exact displacement will be given by the equations previously developed. If the flow rate is less than 2000 cubic feet per hour, the curve will be dis placed toward lower temperatures. The size of these displacements will be about 50 F., depending on how much the flow rate is increased or decreased. In a similar fashion, a change in sheet thickness from the 0.036 inch upon which the curve is based will also cause a shift. For example, increasing the thickness from 0.036 inch to 0.050 inch will displace the curve about 50 F. toward higher temperatures.

FIG. 3 shows experimental data which supports the work cited above. The data were obtained on annealed specimens with the annealing time, hydrogen flow rate, specimen gage, and specimen weight being held constant. Under these conditions the amount of nitrogen removed, as measured by the aging index, passed through a maximum with temperature, thus indicating that an optimum annealing temperature indeed exists. Moreover, the curve itself in FIG. 3 was drawn according to the theoretical predictions. Note how well the experimental data points conform to the predicted behavior as shown by the solid line. These results were obtained in an experiment scaled down 2000 to 1 from a commercial operation.

The experimental data in FIG. 3 was derived from fifteen stacked sheets of steel having the nominal composition given above and having a total weight of 240 grams. Note that the indicated optimum annealing temperature of about 1112 F. is in the upper part of the range 975 F. to 1150 R, which confirms the data shown in FIG. 2 where the optimum annealing temperature increases as the coil mass increases.

The result of open coil annealing coils of rimmed steel under actual manufacturing conditions is shown in Table An open coil anneal was used as the furnace and a flow rate of 2000 cubic feet per hour of hydrogen maintained throughout the anneal. None of the materials annealed under actual manufacturing conditions exhibited any return of the yield point after temper rolling and aging one hour at 212 F. Typical mechanical properties are shown in Table IV.

TABLE IV Angle With Rolling Yield Tensile Elongation Direction Strength Strength in 2" (p.s.i.) (p.s.i.) (percent) Hardness Rb=44/45; Grain Size=ASTM #9.

These are at least as good as a conventional rimmed steel and may, in some cases, be better.

The present invention thus provides a means for denitriding rimmed low carbon steels in the presence of dry hydrogen at a minimum annealing time. It is, however, important to be aware of the following factors: First, the equilibrium between the furnace atmosphere and the surface of the steel must be mainta ned at all times. Second, complete mixing of gases must take place in the furnace to prevent the accumulation of ammonia gas at the surfaces of the steel. Third, the ammonia must obey the ideal gas low; and fourth, the only chemical reaction taking place is the combination of dissolved nitrogen and gaseous hydrogen to form gaseous ammonia. The first assumption merely dictates that the reaction rate of the formation of ammonia at the surface of the sheet is much faster than either the removal of ammonia from the furnace or the solid-state diffusion of nitrogen through the steel. In other words, it is not a rate controlling factor. The second assumption dictates that the velocity of gas moving across the sheet is sufi'iciently high to thoroughly mix the ammonia and hydrogen, and that no short circuiting of hydrogen exists. If this were not true, the ammonia would be exhausted from the furnace at a slower rate than that predicated and the efficiency of the process would suffer accordingly. In a commercial application this means, as a practical matter, that the hydrogen flow rate must be at least 1500 cubic feet per minute for coil sizes in the range of 10,000 to 80,000 pounds, assuming that a conventional batch annealing furnace is employed for coils of this size. The third and fourth assumptions are, of course, self-explanatory.

I claim as my invention:

1. A process for producing non-aging rimmed plaincarbon steel containing above about 0.04% by weight of carbon, said steel having, when cast, a nitrogen content of about 0.004%, said process consisting essentially of the steps of rapidly cooling said steel through the transformation range to cause a fine carbide distribution and then slow-cooling said steel to about 300 F. to allow precipitation so as to prevent the occurrence of carbon strain aging, and then loosely coiling said steel in strip form and annealing said strip in an enclosure in the presence of dry hydrogen in an open-coil box anneal at a temperature in the range of about 975 F. to 1150 F. for a time sufficient to reduce the nitrogen content of the steel in solution to at most 0.0005 by weight, the flow of hydrogen through the enclosure being sufiicient to mix the ammonia gas formed at the surfaces of the strip with hydrogen and prevent any accumulation of ammonia at the surfaces from impeding the reaction of nitrogen in the steel with the hydrogen.

2. A process as defined in claim 1, characterized in that said steel strip is in the form of coils weighing about 40,000 pounds to 80,000 pounds, lower annealing temperatures in said range of 975 F. to 1150 F. being used with heavier coils and higher temperatures with lighter coils.

3. In the process for producing non-aging rimmed steel in strip form, the steps of loosely coiling the strip, placing the coiled strip in an enclosure and causing dry hydrogen to flow through the enclosure, and introducing heat into the enclosure to maintain the temperature of the strip in the range of about 975 F. to 1150 F. for a time sufiicient to reduce the nitrogen content of the steel in solution to at least 0.0005 by weight, the flow rate of hydrogen through the enclosure being suflicient to mix the ammonia gas formed at the surfaces of the strip with hydrogen and prevent any accumulation of ammonia at the surfaces from impeding the reaction of nitrogen in the steel with the hydrogen.

4. In the process for producing non-aging rimmed steel in strip form, the steps of loosely coiling the strip, placing the coiled strip in an enclosure and causing dry hydrogen to flow through the enclosure, and introducing heat into the enclosure to maintain the temperature of the strip in the range of about 975 F. to 1150 F. for a time sufiicient to reduce the nitrogen content of the steel in solution to at least 0.0005 by weight, the flow rate of hydrogen through the enclosure being above 1500 cubic feet per hour.

References Cited UNITED STATES PATENTS 2,271,242 1/ 1942 Altenburger 14816 2,360,868 10/1944 Gensamer 148-16 FOREIGN PATENTS 942,341 11/1963 Great Britain.

OTHER REFERENCES Metal Progress, vol. 37, 1940, pp. 642 and 643 relied on.

CHARLES N. LOVELL, Primary Examiner. 

1. A PROCESS FOR PRODUCING NON-AGING RIMMED PLAINCARBON STEEL CONTAINING ABOVE ABOUT 0.04% BY WEIGHT OF CARBON, SAID STEEL HAVING, WHEN CAST, A NITROGEN CONTENT OF ABOUT 0.004%, SAID PROCESS CONSISTING ESSENTIALLY OF THE STEPS OF RAPIDLY COOING SAID STEEL THROUGH THE TRANSFORMATION RANGE TO CAUSE A FINE CARBIDE DISTRIBUTION AND THEN SLOW-COOLING SAID STEEL TO ABOUT 300*F. TO ALLOW PRECIPITATION SO AS TO PREVENT THE OCCURRENCE OF CARBON STRAIN AGING, AND THEN LOOSELY COILING SAID STEEL IN STRIP FORM AND ANNEALING SAID STRIP IN AN ENCLOSURE IN THE PRESENCE OF DRY HYDROGEN IN AN OPEN-COIL BOX ANNEAL AT A TEMPERATURE IN THE RANGE OF ABOUT 975*F. TO 1150*F. FOR A TIME SUFFICIENT TO REDUCE THE NITROGEN CONTENT OF THE STEEL IN SOLUTION TO AT MOST 0.0005% BY WEIGHT, THE FLOW OF HYDROGEN THROUGH THE ENCLOSURE BEING SUFFICIENT TO MIX THE AMMONIA GAS FROMED AT THE SURFACES OF THE STRIP WITH HYDROGEN AND PREVENT AND ACCUMULATION OF AMMONIA AT THE SURFACES FROM IMPEDING THE REACTION OF NITROGEN IN THE STEEL WITH THE HYDROGEN. 